The method and device according to the present invention correspond to the mechanical solution of the metrological alignment principle known as “Reference Positioning System” (RPS). The principle for placement and alignment according to the RPS, for instance, is described in Lichtenberg, Thilo (2006): “A flexible vehicle measurement system for modern automobile production” (Master Thesis at the Faculty of Engineering, the Built Environment and Information Technology of the Nelson Mandela Metropolitan University).
It is common practice during the production of a car to measure features and properties of its different components. These measurements can be carried out in special measurement cells by means of either contact or non contact measuring gauges, for example based on laser or photogrammetric principles. Such a procedure, for instance, is disclosed in DE 195 44 240 A1.
Large workpieces, especially sheet metal parts, such as car bodies, can have significant production tolerances. The deviation of a car body can be as large as 2 mm in length, whereas the measurement tolerance is comparatively small, for example about 150 μm. This means that the alignment of the workpiece—its mechanical exact positioning and orienting—is a challenging task.
A workpiece, such as a car body, can be very heavy, which means that the generated forces and friction can be very high, especially if the contact angles are not flat and the contact areas are very small. Furthermore, the loading of the part into the measurement cell is either relatively inaccurate or requires high effort and cost.
In many known applications the means by which the alignment of a component is defined cannot be measured during the measurement process itself as throughout the whole measurement process the measured component is held on the very same means. Thus, access to these means is blocked for measuring equipment. Therefore, in these applications it is essential for overall accuracy of any kind of measurement equipment to have the component placed exceptionally accurate and repeatable in a predefined position and orientation.
Partially, the alignment is already taken into consideration in the shape of certain alignment features of the workpiece. Typically, these alignment features comprise a circular hole, an elongated slot and a planar surface.
When designing the positioning units of the reference positioning system, a trade-off between maximum error in the Z-position and the ability of self-centring must be made. This means that a flat geometry causes little error in the Z-position, but makes it unlikely that the workpiece will be centred in the exact XY-position due to geometrical and also physical reasons, such as friction. In case the alignment features are not oriented completely orthogonal to the Z-axis, additional errors may occur. The dimensional tolerances of the alignment features themselves, such as the diameters of the circular hole and the elongated slot, are also critical.
Conventionally, there are two options to address this problem:
In the first option, the support, which for example can be a cylindrical bolt, is smaller than the minimum dimension of the alignment feature. This means that the position of the workpiece can vary within the maximum clearance space which is at least as big as the tolerance of the alignment feature. This tolerance includes the theoretical production variation and wearing effects caused by positioning the part in various assembly steps.
In the second option, the support has the form of a sphere or is conical, which means that it is self-centring in X and Y directions. But, as a consequence, its Z-position cannot be defined accurately.
Deviations in position or dimension of a workpiece's feature compared to its theoretical value are typically expressed by referring to the workpiece's own coordinate system, the Part Coordinate System (PCS). The PCS is defined during the design process of its corresponding workpiece. For measuring a position or dimension of a feature of the workpiece the PCS therefore needs to be connected to the measurement coordinate system, i.e. in the real three-dimensional space.
This alignment procedure preferably can be carried out by means of the “3-2-1 rules”. In case of a statically determined placement of a rigid body the minimum number of constraints is used, which means that the location of one alignment feature is known in three dimensions (XYZ), the second in two (YZ) and the third in one dimension (Z). In other terms, the first location defines three degrees of freedom, the second two degrees of freedom and the third one degree of freedom. Therefore, six degrees of freedom are defined, and so are the position and orientation of a (rigid) workpiece. In the definitions of the workpiece, especially for sheet metal parts, there can be three features defined which serve that purpose, for example a circular hole, an elongated slot and a planar surface.
The part could be placed on conical pins which are movable in the X- and Y-dimensions. Their XY-position is read by measurement means, for instance by a laser tracker or preferably by encoder means, such as linear and/or rotary encoders, and the alignment of the workpiece is done based on those measurement data.
However, in some applications a clamping of the workpiece is required during the measurement, which means that some or all of the alignment features are clamped down of their nominal zero-position in the Z-dimension. In this case floating pins cannot be applied in a relatively uncomplicated way.
Furthermore, conical pins lead to a possibly inaccurate alignment with respect to their longitudinal axis, i.e. the Z-axis of the coordinate system, due to the production tolerance of the feature itself, e.g. of the diameter of its hole. This means that the alignment feature might not be leveled correctly in the Z-dimension.
Additionally, supporting an elongated slot (as typically used at the second feature) with a conical shaped pin causes high pressure at a very small contact area, leading to a wearing off of the material and thus to fewer accuracy.